Physicians frequently use optical devices to facilitate examining an area that might otherwise be difficult to observe. For example, when performing arthroscopic knee surgery, a surgeon inserts an arthroscope through a small incision in the patient's leg and manipulates the arthroscope to various areas of interest. The arthroscope uses optical fibers to illuminate the area of interest inside the patient's knee and an optical assembly for picking up an image and transmitting it either for direct observation by the surgeon or to a video camera for display on a TV monitor. Because the surgeon will be moving the arthroscope to various areas of interest throughout the procedure, the distance between the arthroscope's distal tip and the area of interest will vary. It is therefore necessary to provide some mechanism for focusing the image picked up by the arthroscope in order to produce a high quality view on the monitor. Such a focusing mechanism typically consists of at least one focusing lens whose position can be adjusted by the surgeon during the procedure. The focusing lens is placed in a chamber in order to minimize the intrusion of contaminants and to prevent it from fogging during the procedure. The chamber also protects the lens when the device is being cleaned and sterilized between surgical procedures. Cleaning is typically achieved by washing in soaps and disinfecting solutions. Sterilization is accomplished usually by exposure to ethylene oxide gas or by steam autoclaving (the application of superheated steam under high pressure to the medical instrument). Since it is necessary to move the focusing lens within the chamber to achieve a desired focus, a mechanical adjustment mechanism extends within the sealed chamber and contacts the lens (or some structure supporting the lens) to enable the surgeon to manually move the lens. The point at which the mechanism penetrates the chamber must be sealed as tightly as possible to keep the lens clean and fog-free.